PurpleSkyz

A rare fallstreak hole cloud was observed today over Melbourne, Australia, Meteo Europe team reports. Because of their rarity and unusual appearance, as well as very little exposure in media, fallstreak holes are often mistaken for or attributed to unidentified flying objects.A fallstreak hole, also known as a hole punch cloud, punch hole cloud, skypunch, canal cloud or cloud hole, is a large circular or elliptical gap that can appear in cirrocumulus or altocumulus clouds. Such holes are formed when the water temperature in the clouds is below freezing but the water has not frozen yet due to the lack of ice nucleation particles.When ice crystals do form it will set off a domino effect, due to the Bergeron process, causing the water droplets around the crystals to evaporate: this leaves a large, often circular, hole in the cloud. ​

Image copyright David Barton.

The following image is another example of this phenomenon observed on August 17, 2008 about 20 km south of Linz, Austria by H. Raab: